This application claims the benefit of a Japanese Patent Application No.2000-295840 filed Sep. 28, 2000, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to plastic optical elements, plastic optical element producing methods and plastic optical element producing apparatuses, and more particularly to a plastic optical element which is used in an optical scanning system of a laser digital copying machines, laser printers and facsimile machines or used in an optical equipment such as a video camera, and to a plastic optical element producing method and a plastic optical element producing apparatus for producing such a plastic optical element.
2. Description of the Related Art
In optical elements such as a lens and a prism, a high accuracy is required of the surface shape and internal double refraction. For this reason, conventionally, glass is mainly used for the optical element. Recently however, plastics are used increasingly for the optical element due to the large degree of freedom with which the plastics may be shaped and the high productivity which may be achieved by using the plastics. This trend is supported by the development of resin materials having a low double refraction characteristic, and the improved molding techniques which enable production of a molded product having a highly accurate shape and a low double refraction.
Conventionally, resin materials mainly used for the optical elements include polycarbonate and acrylic resin. However, polycarbonate has a large double refraction, and acrylic resin has a water or moisture absorbing characteristic, thereby limiting the use of such materials for the optical elements. However, resin materials having a low water or moisture absorbing characteristic and a low double refraction have recently been developed, to thereby expand the usage of the resin materials for the optical elements. Such resin materials include Zeonex (product name) manufactured by Nippon Zeon and Arton (product name) manufactured by JSR, for example. In addition, improved molding techniques fill the resin at a low pressure and apply pressure on the entire mold or via an insert to realize an injection molding, to improve the accuracy of the shape of the molded product and to obtain a low double refraction. For these reasons, there is increasing trend to use plastics for the optical elements.
A plastic optical element, such as a plastic scanning lens, which is formed by the injection molding, has a shape with a satisfactory accuracy and a low double refraction. However, a refractive index distribution remains within the molded plastic optical element as indicated by (a) in FIG. 1. Consequently, especially as a high-precision plastic optical element, the optical characteristic that is obtainable is still insufficient and unsatisfactory.
In addition, the refractive index is larger (H: high) towards the surface of the plastic optical element and smaller (L: low) towards the center as indicated by (b) in FIG. 1. For this reason, when the plastic optical element is to form an imaging lens, an error will be generated in the imaging position.
Furthermore, as indicated by (c), (d) and (e) in FIG. 1, the refractive index distribution is also generated at the sub scanning cross section, and this refractive index distribution causes an image surface distortion in the sub scanning beam, that is, a deviation in the focus position. For example, in the case of the optical scanning lens of the laser printer, the refractive index distribution causes the beam spot which is to be converged on a scanning surface to move away from a designed position towards an optical deflector, and the beam spot diameter on the scanning surface becomes larger than a designed value, thereby deteriorating the quality of an image which is written by the optical scanning, as described in a Japanese Laid-Open Patent Application No.10-288749.
The refractive index distribution within the plastic optical element is caused by the following. That is, when molding the resin, a temperature decrease in a vicinity of a wall surface of the mold, that is, at a peripheral portion of the resin, is sharp, but the temperature decrease is gradual at a central portion of the resin. Hence, in a state where the injection filling of the resin is made and the initial applied pressure on the resin is high, the vicinity of the wall surface of the mold is rapidly cooled to solidify the peripheral portion of the resin, and the density of the resin consequently becomes high at the peripheral portion of the resin. But since the applied pressure on the resin is reduced by the time the central portion of the resin is cooled and solidified, the density of the central portion of the resin becomes low. As a result, the density of the resin becomes higher towards the surface of the optical element, and becomes lower towards the central portion of the optical element. Because there is a high correlation between the density and the refractive index, the refractive index becomes larger towards the surface of the optical element, and becomes smaller towards the central portion of the optical element, to thereby generate the refractive index distribution.
The main cause of the refractive index distribution is the rapid cooling of the resin in the vicinity of the wall surface of the mold. Hence, after the resin is injected and filled in a high-temperature mold, it is conceivable to carry out an annealing to gradually cool the resin and reduce the temperature distribution within the molded resin, so that it is possible to obtain a lens having a small refractive index distribution. But according to this conceivable method, a molding cycle becomes extremely long, and the productivity becomes poor to increase the production cost of the optical element.
On the other hand, various methods have been proposed which prescribe the lens shape and use a region of the lens where the refractive index distribution is small. For example, a Japanese Laid-Open Patent Application No.8-201717 proposes an optical scanning unit employing a first method which prescribes the lens shape to satisfy a relationship h/t>2, where t denotes a thickness of the beam in the beam propagating direction and h denotes a height of the beam in a vertical direction to the beam propagating direction. By making the value of h large in this first method, it is possible to reduce the temperature distribution in a region where the beam is transmitted at the time of cooling the resin, and to utilize the small refractive index distribution in this region so as to reduce an error in the imaging position.
In addition, a Japanese Laid-Open Patent Application No.9-49976 proposes a second method which makes an optical design by taking into account the refractive index distribution. According to this second method, it is possible to cope with the error in the imaging position of the imaging lens caused by the refractive index distribution, by modifying the shape of the imaging lens. By shifting the design value of the imaging position towards the rotary polygonal mirror, it is possible to image the beam on the scanning surface.
Moreover, a Japanese Laid-Open Patent Application No.11-77842 proposes a third method which reduces the refractive index distribution during an annealing process. The annealing process heats the resin outside the mold, maintains the resin within a predetermined temperature range for a predetermined time, and then cools the resin. According to this third method, it is possible to reduce the refractive index distribution within the optical element.
However, according to the first method, a region through which the beam is not transmitted, that is, a portion other than an effective region of the optical element, increases. As a result, an mount of resin which is required to form the optical element increases, and it is necessary to make the molding cycle (cooling time) long in order to prevent shrinkage. For this reason, the production cost of the optical device becomes high, and the shape of the optical element such as the lens becomes limited, thereby limiting the degree of freedom with which the optical element may be designed.
According to the second method, the mold is made, the optical element such as the lens is then evaluated after determining the molding conditions to suit the mold, and a shape correcting value is determined thereafter. Consequently, if it becomes necessary to modify the molding conditions due to other inconveniences of the molding, the shape correcting value must be modified accordingly, and a mirror surface insert needs to be remade. Furthermore, when making a mold made up of a large number of dies, the shape correcting value must be changed for each cavity, and it is necessary to create a number of processing programs corresponding to the number of cavities. Accordingly, a number of tries of the molding becomes extremely large, and the number of mirror surface inserts which need to be made increases, to thereby increase the production cost of the optical element.
Moreover, the third method is more effective compared to the first and second methods in that the refraction index distribution can be reduced to a certain extent by the annealing process which takes a short time. However, the third method cannot completely eliminate the retraction index distribution. For this reason, as the precision required of the optical element increases, it becomes necessary to further reduce the refractive index distribution.